This invention relates to the field of data compression. More particularly, the invention relates to data compression associated with image data storage or transmission, especially in connection with two-dimensional image coding involving data compression of image data representing screened images which utilizes vertical mode coding. Specifically, the invention is directed to a method and apparatus for block coding image data during the process of being coded utilizing a vertical mode coding technique, thereby providing enhanced data compression in order to further economize image data storage or transmission requirements.
In one aspect, for example, the invention relates to image data transmission. Equipment for telecommunication of two-dimensional images is known. The images can comprise text, as well as pictorial information, such as illustrations, photographs, and graphic information. Known telecommunication equipment comprises a system whereby a transmitter at one location encodes the image, the encoded image data is communicated over a communication link, such as a telephone line, to a receiver at another location, and the receiver decodes the image data. Consequently, a facsimile of the original image is communicated over a distance from the transmitter to the receiver. Hence, the telecommunication equipment has become known as facsimile transmission equipment.
Facsimile transmission equipment is commercially available from various manufacturers. In order to provide compatiblity among the facsimile transmission equipment constructed by different manufacturers, various standards have been adopted. Known facsimile transmission equipment is typically standardized for providing coding procedures as described in "STANDARDIZATION OF GROUP 3 FACSIMILE APPARATUS FOR DOCUMENT TRANSMISSION," CCITT Recommendation T.4 (Geneva, 1980), the disclosure of which is hereby incorporated by reference herein in its entirety. These coding procedures include one-dimensional image coding and two-dimensional image coding.
Whether or not one-dimensional or two-dimensional image coding is utilized is defined by the known CCITT facsimile coding procedures. These CCITT facsimile coding procedures also specify conditions under which various coding modes may be utilized in connection with two-dimensional image coding. If a current scan line alone is used for image coding, the process is referred to as horizontal mode coding. If both a reference scan line and the current scan line are used for image coding, the process is referred to as vertical mode coding.
By way of comparison, the horizontal mode coding technique does not require a reference scan line in connection with coding a scanned line of the image. In contrast, the vertical mode coding technique requires a reference scan line for coding a scanned line of the image as the image data is encoded for transmission at the transmitter. Also, the reference scan line is required by the vertical mode coding technique when the transmitted image data is decoded at the receiver.
Typically, the major cost for operation of known facsimile transmission equipment is the cost for use of the communication link, such as a telephone line. The cost for the use of the communication link is based on the amount of image data which is communicated.
Preferably, the image data is compressed by means of the known CCITT facsimile data compression coding algorithms utilizing the vertical mode coding technique prior to facsimile transmission. The known vertical mode coding technique utilizes a Delta Modulation scheme, whereby the current scan line to be coded is represented by coding only the changes or differences between the current scan line and the reference scan line and the result or difference data comprises the image data which is transmitted. The purpose of the data compression is to reduce the amount, more particularly, the number of bits, of image data to be transmitted, thereby reducing the operating cost. Presently, the amount of image data to be transmitted is of particular concern in the case of pictorial information.
Known facsimile transmission equipment generally requires that a pictorial image be screened prior to coding and transmitting the image. Screening a pictorial image can be performed in several manners.
Screened pictorial images under a magnifying glass generally comprise either circular or diamond-shaped dots aligned in vertical columns. The size of each circle or diamond, or the density of circles or diamonds, in a given area of the pictorial image varies as a function of the gray scale for the given area.
Unfortunately, if a screened pictorial image is to be transmitted, the known vertical mode coding technique does not operate well, because this technique cannot optimally utilize the typical Delta Modulation scheme which is employed. Invariably, the typical Delta Modulation scheme for compressing the current scan line to be coded uses the immediately preceding scan line as the reference scan line, and the current scan line to be coded is represented by coding only the changes or differences between the current scan line to be coded and this immediately preceding scan line. However, the differences between the current scan line to be coded and the immediately preceding scan line can be great. Consequently, the amount of image data to be transmitted over the communication link can be substantial.
Typically, the amount of image data transmitted is not presently reduced by any other technique. This translates to a high cost for operation of the facsimile transmission equipment, since the cost for the use of the communication link is determined by the amount of image data communicated.